Remove a character from a table read.

Removes ONE character from a table read's cast roster, so it can be set up again. Clears the character's voice from the read and its finalized flag (keeping the rest of its profile) and clears its canonical portrait on the script. The read is resolved from scriptUploadId (the caller's latest read for that script; a script that is not yours or has no read is 404 script_not_found). An unknown field, or a character that is not a non-empty string, is refused with 400 validation_failed — the same validation the Writers Lab's cast roster, MCP remove_character and sleeperhit script table-read remove-character go through. Free and naturally idempotent: removing a character the read no longer has is a no-op, and still answers removed. Scope: story:write.